Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefit of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices, such as, for example, a urea based Selective Catalytic Reduction (SCR) catalyst, in which NOx is continuously removed through active injection of a reductant, such as urea, into the exhaust gas mixture entering the catalyst. A typical lean exhaust gas aftertreatment system may also include an oxidation catalyst which is coupled upstream of the SCR catalyst to convert hydrocarbons and carbon monoxide in the exhaust gas mixture. The inventors herein have recognized that one way to achieve fast SCR catalyst warm-up and thus improve its NOx conversion efficiency is to increase the hydrocarbon concentration in the exhaust gas mixture entering the oxidation catalyst, which may be accomplished by either external hydrocarbon (fuel) injection or by adjusting engine-operating conditions, such as injection timing. The extra hydrocarbons will burn in the oxidation catalyst and the heat so generated will warm up of the SCR catalyst.
The inventors herein have further recognized that in such a system, soot and unreduced hydrocarbons from the oxidation catalyst may adsorb on the surface of the SCR catalyst, thus contaminating the SCR catalyst and reducing its NOx conversion efficiency. The inventors have also recognized that maintaining it at a temperature above the boiling point of hydrocarbons, or the regeneration temperature of soot, for a predetermined amount of time, can regenerate the SCR catalyst. Additionally, the inventors have recognized that NOx conversion in the SCR catalyst is improved in the presence of stored ammonia. Therefore, since the regeneration process causes some of the ammonia stored in the SCR catalyst to be released, NOx conversion efficiency of the SCR catalyst following regeneration will be less than optimal.
As will be described below, a method for regeneration of an SCR catalyst is disclosed which removes soot and hydrocarbon contamination while maintaining ammonia storage in the catalyst required for optimal NOx conversion efficiency.